Drill steel centralizer for extension drill steels



June 1967 G. K. A. NILSSON DRILL STEEL CENTRALIZER FOR EXTENSION DRILL STEELS 2 Sheets-Sheet 1 Filed July 6, 1964 I VENTOR Gunnflfl Kuaf Hlf Nilsson United States Patent 3,328,052 DRILL STEEL CENTRALIZER FOR EXTENSION DRILL STEELS Gunnar Kurt Alfred Nilsson, Nacka, to Atlas Copco Aktiebolag, Naeka, tion of Sweden Filed July 6, 1964, Ser. No. 380,420 Claims priority, application Sweden, July 10, 1963, 7,673/ 63 Claims. (Cl. 28524) This invention relates to drill steel centralizers and coupling aids to be mounted on the feed shell of a rock drill for cooperating for coupling and uncoupling purposes with the coupling sleeves of a string of extension drill rods actuated by the rock drill and connected by coupling sleeves wider than the drill rods. In connection with extension drill steel equipment for rock drills it has already been suggested to use the usual drill steel centralizer provided at the forward end of the feed shell and performing the task of guiding the drill string during its axial movement, also as an aid during the coupling and uncoupling operations, whereby drill rods are added to or removed from the drill string usually by screwing or unscrewing the sleeves and drill rods thereof. To attain such additional task for the centralizer, means can be included therein providing an engagement between the centralizer and the string, such engagement preventing or resisting rotation of the drill string outside .of the feed shell. -By using conventional rock drills equipped with rotation motors capable of rotating bi-di-rectionally depending on control, the drill rods, and sleeves engaged by the rock drill On the feed shell can be threaded out of or into the portions of the string immobilized by the centralizer. The centralizer designs hitherto presented in connection herewith, however, are working on the premise of either a time consuming manual handling of the drill steel centralizer for attaining the desired functions or the use of relatively costly and bulky equipment when mechanization of the drill steel centralizer operation is desired. In conventional solutions no automation of the drill steel centralizer function has hitherto been presented. It is an object of the invention to provide a drill steel centnalizer which by means of a minimum of simple details makes the manoeuvering of the centralizer rapid and convenient during downward drilling and coupling of the drill rods as well as during drilling in upward direction and uncoupling of the rods. A further important object of the invention is to fully automatize the drill steel centralizer function during uncoupling of the drill rods in connection with raising of the extended drill string from a downwardly pointing drill hold or in connection with coupling of the drill rods when one is drilling in upward direction.

The above and other objects of the invention will be come obvious from the following description and from the accompanying drawings, in which three embodiments of the invention are illustrated by way of examples. It should be understood that these embodiments are only illustrative of the invention and by no means limit the scope thereof as the invention may be modified further in several different ways within the scope of the appended claims.

FIG. 1 shows a side view of the drill steel centralizer according to the invention during drilling in downward direction. FIG. 2 is a top plan view on the line 22 in FIG. 1. FIG. 3 is a top plan view corresponding to FIG. 2 but showing the drill steel centralizer in frictional engagement with a coupling sleeve. FIG. 4 shows a section on the line 44 in FIG. 2. FIG. 5 illustrates in a view corresponding to FIG. 4 the passage of a coupling sleeve through the drill steel centralizer. FIG. 6 shows a section Sweden, assignor Sweden, a corporaon the line 6-45 in FIG. 3. FIG. 7 is a partly sectional 3,328,052 Patented June 27, 1967 top plan view of a modified embodiment. FIG. 8 is a sectional view on the line 8-8 in FIG. 7. FIG. 9 is a sectional view corresponding to FIG. 8 of a still further modification. FIG. 10 finally is an enlarged section-a1 view on the line 10 10 in FIG. 9.

On the forward end of a feed shell 10 diagrammatically indicated in FIG. 1 is fastened by means of bolts a bracket 11 carrying two parallel pivots 12 extending in the longitudinal direction of the feed shell 10. The housing of the drill steel centralizer is divided and consists of two halfcylindrical rings 13 and 14. The ring halves 13 and 14 are pivotally arranged each about one of the pivots 12 and are for that purpose carried by arms 15 afiixed to hubs 16 journalled on the pivots 12. At their ends turned away from the pivots 12 the ring halves 13, 14 carry cars 17 and 18 through which a bolt 19 is inserted. The bolt 19 also passes through a spacing sleeve 20 placed between the ears 17, 18 against which the ears are pressed by means of a spring 21. The spring 21 bears against the outside of the ear 17 and the inside of a nut 22 which is threaded on the bolt 19. By axial adjustment of the nut 22 on the bolt 19 the tension of the spring 21 can be adjusted.

The ring halves 13, 14 are each provided with spaced bosses 23 between which pivots 24 extend in substantially parallel relation to the plane of division of the ring halves 13, 14. About the pivots 24 semicircular lids 25 are pivotally journalled on the ring halves 13, 14. The lids 25 can be pivoted into a common plane transversely and preferably perpendicularly to the longitudinal axis of the feed shell 10 and to the drill string, and in such position the lids 25 are resting against abutments 26, FIG. 5, on the ring halves 13, 14. The lids 25 are constantly urged towards the abutments 26 and the said perpendicular plane by a biasing force which in the embodiment shown is represented by the force of gravitation. The lids 25 are provided at their center with jaws 27 each consisting of a semi-cylindrical body with semi-cylindrical centering surfaces 28 thereon somewhat wider in diameter than the drill rods. In the adjacent position of the jaw-s 27, in which the lids 25 are disposed in said perpendicular plane, the jaws 27 are adapted to guidingly surround the drill rods 29 to be guided by the drill steel centralizer.

The drill rods 29 are in the usual manner provided with threads 30 at their ends and are kept together by coupling sleeves 31. As an alternative instead of the threads there may be provided cooperating bayonet lock members on the sleeves 31 and drill rods 29 to effect the coupling and uncoupling therebetween. It is understood that the rock drill, which is movable to and fro along the feed shell 10 by machine power from a feeding motor and which can be a percussive drill or a rotary drill, is arranged to rotate the connected drill rods 29 bidirectionally. The rotation and the impact energy can be transmitted from the drill to the drill string in the usual manner by means of a shank adapter, not shown, mounted in the rock drill which adapter is releasably connected or threaded fast to a coupling sleeve 31 on the rear end of the drill string by suitably rotating the shank adapter in the rock drill. An axial shoulder 32 lies centrally in the jaws 27 and forms a transition from the centering surfaces 28 to semi-cylindrical gripping surfaces 33 which are somewhat smaller in diameter than the coupling sleeves 31 and in the adjacent position of the jaws 27 form a socket adapted to receive the sleeves 31 therein. The gripping surfaces 33 are terminated at one end of the jaws 27 by conical entering surfaces 34.

Let it be supposed that a hole has been drilled to the full depth with a percussive drill with the drill string having been built up successively by a number of coupled drill rods 29. With the drill string resting against the bottom of the hole or hanging in the rock drill some additional impacts are delivered to the drill string while relieved from load for loosening the tightly drawn threaded connections therein. T-hereupon raising of the drill string is performed with the drill steel centralizer in the position shown in FIG. 4, in which the drill rod 29 disposed at the opening of the drill hole by means of the uppermost coupling sleeve 31 is affixed to the shank adapter, not shown, and is guided between the centering surfaces 28 of the jaws 27. Due to the rock drill being retracted by the feed motor along the feed shell the drill string is raised. Prior to the rock drill having been retracted to the extent of a full feed length the next coupling sleeve 31 will engage the underside of the jaws 27 turning them and the lids 25 about the pivots 24 in upward direction to their separated position depicted in FIG. 5 in which the sleeve 31 can pass the jaws 27 axially. As soon as the coupling sleeve has passed the jaws 27, the latter will fall down against the abutments 26 because of the force of gravitation to a position in the plane perpendicular to the drill string, i.e. a position corresponding to the starting position depicted in FIG. 4, at

which instant the coupling sleeve 31 will be immediately above the jaws 27. The direction of feed is now reversed and the string is lowered until the coupling sleeve 31 has penetrated in between the entering surfaces 34 and has pressed the guiding jaws 27 radially outwardly taking up an abutting position against the shoulder 32, FIG. 6. Since the gripping surfaces 33 have a somewhat smaller diameter than the coupling sleeves 31, such penetration means that the coupling sleeve will push the lids away from each other against the action of the spring 21 whereby a frictional engagement is created between the gripping surfaces 33 and the coupling sleeve 31. By adjustment of the spring tension in the spring 21 this frictional engagement is chosen of such strength that one in the position according to FIG. 6, by power rotation of the shank adapter can thread the adapter out of the uppermost coupling sleeve 31, or, depending on varying friction in the connections, can thread the uppermost rod 29 in unison with its sleeve 31 out of the coupling sleeve 31 in the centralizer. The frictional grip of the jaws 27 is amplified by friction between the sleeve 31 and the shoulder 32. Unscrewing between the uppermost rod 29 and the sleeve 31 in the centralizer or, as the case may be, between the shank adapter and the uppermost sleeve 31 and rod 29 sitting thereon, is then performed manually by hand tools. Upon removal of the unscrewed d-rill rod 29 and the uppermost coupling sleeve 31, the drill is advanced along the feed shell, whereupon the adapter is threaded by power into the coupling sleeve 31 locked in the centralizer. Thereupon the drill string is again raised a full feed length, whereby the next coupling sleeve 31 passes the drill steel centralizer and in an analogous way is locked automatically therein. The automatic action of the drill steel centralizer is repeated until the entire drill string has been raised.

During downward drilling the drill steel centralizer is used in a position analogous with the position in FIG. 4 firstly for guiding of the first drill rod with the drill bit there-on during collaring of the hole and thereupon for coupling the sleeves 31 by power from the rock drill. By means of a depressable foot pedal 35, which is pivotally journalled on a sleeve 36 on the spacing sleeve 20 and carries a curved rod 37 pointing from below towards the lids 25, and which furthermore at its end carries pieces of pipe 38 projecting obliquely in lateral direction for purposes of engaging the lids, the lids can be opened when necessary to such an extent that the coupling sleeves 31 can pass freely in downward as well as upward direction through the drill steel centralizer. Insertion and coupling of new drill rods during downward drilling is performed by firstly driving the shank adapter carrying the drill string and the uppermost coupling sleeve 31 down into the guiding jaws according to FIG. 6, whereupon the adapter is threaded loose .out of the frictionally gripped sleeve 31 and together with the rock drill is retracted by the feed motor. Thereupon the next drill rod 29 is inserted in the locked coupling sleeve 31 and simultaneously therewith the adapter is screwed into the coupling sleeve 31 loosely screwed on the rearend of the newly inserted drill rod 29. After screwing the parts together the rock drill raises the drill string and the locked coupling sleeve 31 in upward direction from the position in FIG. 6 to a sufficient height for enabling the lids 25 to be swung in upward direction by actuation of the foot pedal 35, so that the now released coupling sleeve 31 during downward drilling freely can pass the centralizer. The coupling operation is repeated when the coupling sleeve 31 adjacent the shank adapter during drilling has been fed down to the level of the jaws 27 and their gripping surfaces 33.

When use of the drill steel centralizer is unnecessary the lids 25 can be turned out into inactive position indicated at 40 in FIG. 2 by dash and dot lines at which instant the entire opening inside of the ring halves 13, 14 is left free. If the drill hole is drilled obliquely it may be justified to provide the biasing force for returning the lid halves 25 against the abutments 26 by means of a pair of torsion springs 411 of which one is diagrammatically indicated in FIG. 2.

It is readily seen that the jaws by mounting the drill steel centralizer upside down on the pivots 12 with respect to the position in FIG. 1 can be induced during drilling in upward direction to grip and to hold the drill string automatically while coupling of new drill rods is in progress. For purposes of uncoupling the drill string subsequent to drilling in upward direction, a stay 42 or a wire may be coupled to the foot pedal 35 so that the guiding jaws can be opened and closed at will from the ground when the coupling sleeves are to be locked or have to pass.

Thanks to the fact that the jaws in their adjacent as well as separated position are disposed at one side of the plane of the lids 25 perpendicular to the axis of the drill string, the jaws evidently form an automatic one way slip device with uni-directional locking action for cooperation with the coupling sleeves 31, which device permits the coupling sleeves 31 to pass through the jaws 27 when the drill string moves in its one axial direction, but which during movement of the drill string in opposite direction automatically locks the next coupling sleeve 31 in the drill string advanced to the drill steel centralizer against rotation and axial movement. During such operation the coupling sleeves 31 are locked by the gripping surfaces 33 frictionally against rotation so that overloading of the drill steel centralizer is prevented if the loosening of the coupling sleeves by impacts prior to uncoupling was not entirely successful. On the other hand the drill string is locked by positive engagement by the abutment shoulder 32 against axial movement so that the string during downward drilling is unable to fall down into the hole or during upward drilling cannot fall out of the drill hole and cause the operator to get hurt.

In the embodiment according to FIG. 7 a single integral ring 39 replaces the ring halves 13, 14 and carries the ears 23 in which the pivots 24 of the lid halves 25 are journalled. Between the pivots 24 and the ears 23 are provided resilient bushings 41 of rubber or plastic material. The ring 39 is connected to the hub 16 journalled on the pivots 12 by means of a single connecting plate 421. At its end turned away from the pivots 12 the ring 39 carries fixedly thereon two parallel ears 43 in which a transverse axle 44 is journalled. On the transverse axle is mounted the spacing sleeve 20 as well as the bushing 36 of the foot pedal 35 as before. A pair of abutments 45 are provided on a suitable level on the inside of the ring 39 in diametrically opposite positions, which abutments coact with the undersides of the lids 25 forming end stops for said lids when they are in a pOsitiOn perpendicular to the drill string, FIG. 8. The operation of the embodiment according to FIGS. 7, 8 is unaltered as for the embodiment of FIG. 1-6 with the exception that the radial gripping act-ion of the guiding jaws 27 results from compression of the bushings 41 instead of compression of the spring 21.

In the embodiment according to FIGS. 9, 10 the height of the abutments 45 is smaller and a prismatic rubber strip 46 is fastened on each abutment 45 coacting with the underside of the lids 25. The gripping surfaces 47 of the jaws 27 are toothed by suitably provided grooves. When one of the coupling sleeves 31 exerts an axial force against the shoulders 32 in the jaws 27, the lids 25 thanks to the rubber strips 46 can be pushed past their normal transverse position, FIG. 9 at which instant the teeth on the surfaces 47 due to the eccentric disposition of the jaws 27 with respect to their pivots 24 exert a frictional grip against the sleeve 31 acting against the shoulders 32, whereby rotation of the sleeve is effectively resisted. Suitable abutments 48 can be provided on the lids 25 coacting with the upper side of the ring 39, whereby the lids 25 are prevented from being pushed too far from the horizontal position. This prevents an overcharging of the jaws 27 and pivots 24 during impacts which occasionally can be delivered to the shoulders 32 when the uppermost threaded connections are to be hit loose with the drill string in position in the drill steel centralizer. The bias for urging the jaws 27 towards the adjacent position with the lids substantially perpendicular to the drill string can as an alternative be provided solely by springs similar to the spring-s 411 indicated in FIG. 2.

What I claim is:

1. In drill steel centralizer apparatus of the character described for mounting on the feed shell of a rock drill for accommodating the coupling sleeves of a string of extension drill rods actuated by said rock drill and connected by coupling sleeves larger than said rods for the coupling and uncoupling thereof, and having a support mounted on the feed shell with jaws movably mounted between a closed and a separated position on said support and forming in said closed position thereof an axially divided socket in alignment with said string, said socket having an opening sufiicient to receive said sleeves therein upon axial movement of said string in one longitudinal direction thereof, the combination which comprises means for biasing said jaws to said closed position thereof, resilient means on said support and connected to said jaws and responsive to said socket thereon for receiving a sleeve in said closed position of said jaws for causing said socket to grip said sleeve frictionally and resist rotation during said coupling and uncoupling thereof, axial abutment means on said jaws adjacent said socket, said abutment means cooperating in said closed position with the inner end of the sleeve received in said socket for positively preventing said sleeve from passing said jaws axially in said one direction, guide surfaces on said jaws forming in said closed position thereof a guiding opening for said drill rods larger than said rods but substantially smaller than said sleeves, and means connected to said jaws for the movement thereof to said separated position for permitting said sleeves to freely pass said jaws during axial movement of said string in either direction.

2. A drill steel centralizer as :set forth in claim 1 in which said jaws are disposed at the side of a transverse plane opposite to the direction of the force of gravitation whereby said jaws are biased towards said closed position.

3. Drill steel centralizer apparatus as set forth in claim 1 in which is provided means on said support providing a certain movability of said jaws toward and away from said string in directions transverse thereto, with said resilient means on said support for constantly urging said jaws toward said string in said directions transverse thereto,

and cooperating means at the mouth of said socket and on said sleeves for expanding said jaws transversely against the action of said resilient means for enabling said sleeves to be gripped frictionally by said socket when a sleeve is received fixedly therein.

4. A drill steel centralizer as set forth in claim 1 in which said axial abutment means are formed by the transition between said socket and a bore provided in said jaws in coaxial adjacency to said socket, said bore being somewhat wider than said drill rods for guiding said drill rods through said socket during axial movement of said string.

5. Drill steel centralizer apparatus as described in claim 1 in which said resilient means is disposed between said support and said jaws axially at the end of said jaws opposite to said socket for permitting said jaws with said abutment means thereon engaged by a sleeve to move axially in said one direction against the action of said resilient means a limited distance past said closed position of said jaws so that said socket, due to its mounting at one side of said transverse plane, is contracted transversely of said string to frictionally grip said sleeve as received in said socket.

6. Drill steel centralizer apparatus as described in claim 1 in which said support has disposed thereon ring halves with each half carrying one of said jaws and surrounding said string, pivot means on said feed shell substantially parallel with said string and connected to one end of said halves for pivotally moving them with respect to said string, and with said resilient means including a spring disposed on the ends of said halves opposite said pivot means for urging said ring halves toward each other.

7. In drill steel centralizer apparatus of the character described for mounting on the feed shell of a rock drill for accommodating the coupling sleeves of a string of extension drill rods actuated by said rock drill and connected by coupling sleeves larger than said rods for the coupling and uncoupling thereof, and having a support mounted on the said feed shell with jaws movably mounted between a closed and a separated position on said support and forming in said closed position thereof an axially divided socket in alignment with said string, with the diameter thereof being smaller than said sleeves, the combination which comprises resilient means connected to said jaws in said closed position thereof providing radial movability of said jaws toward and away from each other on said support transversely of said drill string and for constantly urging said jaws toward each other transversely of said string, cooperating means in said socket and on said sleeves for expanding said jaws transversely against the action of said resilient means for enabling any of said sleeves to penetrate into and to be frictionally gripped by said socket as a result of movemen of said string in one axial direction thereof, axial abutment means on said jaws adjacent said socket, said abutment means cooperating in said closed position with the inner end of the sleeve received in said socket positively to prevent said sleeve from passing said jaws axially in said one direction, guide surfaces on said jaws for forming in said closed position thereof a guiding opening for said drill rods larger than said rods but substantially smaller than said sleeves, and means connected to said jaws for the movement thereof to said separated position for permitting said sleeves and rods freely to pass said apparatus during axial movement of said string.

8. In drill steel centralizer apparatus of the character described for mounting on the feed shell of a rock drill for accommodating the coupling sleeves of a string of extension drill rods actuated by said rock drill and connected by coupling sleeves larger than said rods for the coupling and uncoupling thereof and having a support on said freed shell with jaws pivotally movable thereon between a closed and a separated position on said support and forming .in said closed position thereof an axially divided socket in alignment with said string, said socket having an opening sufiicient to receive said sleeves therein upon axial movement of said string .in one longitudinal direction thereof, the combination which comprises means for biasing said jaws to said closed position thereof, means disposed on said jaws for providing in the said closed position thereof a limited axial movement in said one longitudinal direction, resilient means disposed between said support and said jaws for resisting said axial movement, abutment means on said jaws engageable by said sleeves for moving said jaws axially corresponding to their axial movability in said one direction as a result of movement of said string in said one direction, pivotally mounted means for said jaws on said support responsive to said additional axial movement of said jaws for contracting said socket radially inwardly for frictionally clamping said sleeves therein, guide surfaces on said jaws forming in said closed position thereof a guiding opening for said drill rods larger than said rods but substantially smaller than said sleeves, and means connected to said jaws for said movement thereof to said separated position for permitting said sleeves to pass freely said apparatus during said axial movement of said string.

9. In drill centralizer apparatus of the character described for mounting on the feed shell of a rock drill for cooperating with a drill string of extension drill rods actuated by said rock drill and connected in aligned disposition by coupling sleeves larger than said rods and having a support mounted on said feed shell, the combination which comprises a pair of cooperating relatively movable jaws pivotally mounted on said support to pivot between a closed and a separated position about axes disposed in a plane transverse to said string for forming in said closed position thereof a socket in alignment with said string with an opening sufficient freely to pass said drill rods axially therethrough but to receive and arrest said sleeves therein upon axial movement of said string in one longitudinal direction, said jaws being disposed in said closed position as well as said separated position on only one side of said transverse plane, the combination which comprises biasing means connected to said jaws for the pivoting thereof toward said closed position whereby said jaws and socket 8 form a one-way slip device for said sleeves permitting them to pass said jaws only when said string moves in a direction opposite to said one longitudinal direction, a bore in said jaws larger than said drill rods and coaxially adjacent said socket for guiding said drill rods during said axial movement of said drill string, axial abutment means at the transition between said socket and said bore for axially arresting said sleeves in said socket in said closed position of said jaws and upon movement of said string in said one longitudinal direction, resilient means connected to said jaws and responsive to said socket in said closed position of said jaws when they are axially receiving a sleeve for causing said socket to grip said sleeves frictionally and resist rotation during the coupling and uncoupling thereof, and means connected to said jaws for pivoting said jaws to said separated position for permitting said sleeves freely to pass said jaws without regard to the direction of movement of said string.

10. Drill steel centralizer apparatus as described in claim 9 in which said socket is smaller than said sleeves, with means disposed on said support providing a certain movability of said jaws radially toward and away from said string, said resilient means constantly urging said jaws radially toward said string, and cooperating surfaces at the mouth of said socket and on said sleeves for expanding said ja'ws transversely against the action of said resilient means for causing said sleeves to be gripped frictionally by said socket when a sleeve is received therein.

References Cited UNITED STATES PATENTS 221,162 11/1879 Edelen 285143 322,242 7/1885 Berry 285143 865,872 9/1907 Cranston 285-143 888,165 4/1908 Harrington 285-322 X 1,485,308 2/1924 Stegeman 308 -3.9 1,486,035 3/ 1924 Rich 24-249 2,010,284 8/1935 Wickersham 285143 X CARL W. TOMLIN, Primary Examiner.

D. W. AROLA, Assistant Examiner. 

1. IN DRILL CENTRALIZER APPARATUS OF THE CHARACTER DESCRIBED FOR MOUNTING ON THE FEED SHELL OF A ROCK DRILL FOR ACCOMMODATING THE COUPLING SLEEVES OF A STRING OF EXTENSION DRILL RODS ACTUATED BY SAID ROCK DRILL AND CONNECTED BY COUPLING SLEEVES LARGER THAN SAID RODS FOR THE COUPLING AND UNCOUPLING THEREOF, AND HAVING A SUPPORT MOUNTED ON THE FEED SHELL WITH JAWS MOVABLY MOUNTED BETWEEN A CLOSED AND A SEPARATED POSITION ON SAID SUPPORT AND FORMING IN SAID CLOSED POSITION THEREOF AN AXIALLY DIVIDED SOCKET IN ALIGNMENT WITH SAID STRING, SAID SOCKET HAVING AN OPENING SUFFICIENT TO RECEIVE SAID SLEEVES THEREIN UPON AXIAL MOVEMENT OF SAID STRING IN ONE LONGITUDINAL DIRECTION THEREOF, THE COMBINATION WHICH COMPRISES MEANS FOR BIASING SAID JAWS TO SAID CLOSED POSITION THEREOF, RESILIENT MEANS ON SAID SUPPORT AND CONNECTED TO SAID JAWS AND RESPONSIVE TO SAID SOCKET THEREON FOR RECEIVING A SLEEVE IN SAID CLOSED POSITION OF SAID JAWS FOR CAUSING SAID SOCKET TO GRIP SAID SLEEVE FRICTIONALLY AND RESIST ROTATION DURING SAID COUPLING AND UNCOUPLING THEREOF, AXIAL ABUTMENT MEANS ON SAID JAWS ADJACENT SAID SOCKET, SAID ABUTMENT MEANS COOPERATING IN SAID CLOSED POSITION WITH THE INNER END OF THE SLEEVE RECEIVED IN SAID SOCKET FOR POSITIVELY PREVENTING SAID SLEEVE FROM PASSING SAID JAWS AXIALLY IN SAID ONE DIRECTION, GUIDE SURFACES ON SAID JAWS FORMING IN SAID CLOSED POSITION THEREOF A GUIDING OPENING FOR SAID DRILL RODS LARGER THAN SAID RODS BUT SUBSTANTIALLY SMALLER THAN SAID SLEEVES, AND MEANS CONNECTED TO SAID JAWS FOR THE MOVEMENT THEREOF TO SAID SEPARATED POSITION FOR PERMITTING SAID SLEEVES TO FREELY PASS SAID JAWS DURING AXIAL MOVEMENT OF SAID STRING IN EITHER DIRECTION. 